1. Field of the Invention
The present invention relates to a composite oxide, a composite oxide carrier and a composite oxide catalyst.
2. Description of the Related Arts
In the case where three-way catalysts for reducing exhaust gases are exposed to a high temperature atmosphere, the oxygen storage capacity (hereinafter will be merely called OSC) achieved by ceria therein decreases. This is caused by sintering of active catalytic elements such as platinum (Pt) or palladium (Pd), and a promoter such as ceria, oxidation of the active catalytic metals, dissolution of rhodium (Rh)to a catalyst carrier, or the like. Furthermore, in the case where catalysts have low OSC, the active catalytic metals tend to be exposed to a changing atmosphere and consequently, the deterioration (coagulation and dissolution) of the active catalytic metals (platinum, palladium and rhodium) may be unfavorably accelerated.
Japanese patent application laid-open No. Hei 4-4043 discloses an oxide carrier in which alumina (Al.sub.2 O.sub.3), ceria (CeO.sub.2) and zirconia (ZrO.sub.2) are co-precipitated to disperse zirconia between the other elements to solve the above problem. And Japanese patent application laid-open No. Hei 7-300315 discloses an oxide carrier in which cerium and zirconium ions are precipitated by adding electrified particles of alumina thereto. However, by using the co-precipitation method and mixing ratio disclosed herein, the dispersing characteristics of the component elements were not so enhanced as to improve OSC and the ratio of purification.
Japanese patent application laid-open No. Hei 7-315840 discloses an aluminum oxide-cerium oxide-zirconium oxide containing compound carrier which is formed by mixing a precipitate obtained from a solution containing a cerium salt and a zirconium salt with alumina powder. However, by the method disclosed therein, composite oxide carriers of which component elements exhibit high dispersing characteristics, cannot be manufactured.
Furthermore, carriers prepared by impregnating alumina powder with a solution containing a cerium salt and a zirconium salt exhibit high dispersing characteristics of a cerium oxide and a zirconium oxide, but the composite (solid solution) of the cerium oxide and the zirconium oxide is insufficient.
Another catalyst including a carrier obtained from a mixture of active alumina powder and ceria-zirconia composite oxide powder, which adsorbes and carries noble metals, has been also known. The catalyst thus arranged was, however, insufficient in OSC at low temperatures.